Solid area development in xerography employing an insulating screen in the charging step



Oct. 6, 1970 G. c. BHAGAT 3,532,494

SOLID AREA DEVELOPMENT IN XEROGRAPHY EMPL SCREEN IN THE C GING S Filed Sent. 1969 NG AN INSULATING INVENTOIgr.

T GOPAL C. BHAGA FIG. 4

A 7' TORNEV United States' Patent Oice 3,532,494 Patented Oct. 6, 1970 3,532,494 SOLID AREA DEVELOPMENT IN XEROGRAIHY EMPLOYING AN INSULATING SCREEN IN THE CHARGING STEP Gopal C. Bhagat, 191 Gregory St., Rochester, N.Y. 14620 Continuation-in-part of application Ser. No. 544,915, Apr. 25, 1966.. This application Sept. 8, 1969, Ser. No. 856,038

Int. Cl. G03g 13/22, 15/00; H011' 37/26 U.S. Cl. 96--1 5 Claims ABSTRACT OF THE DISCLOSURE A method for enhancing solid area development in an electrostatic machine by selectively charging a multiplicity of very small discrete areas of a light sensitive surface prior to exposure of an image to be reproduced.

This is a continuation-in-part of application Ser. No. 544,915, filed Apr. 25, 1966, now abandoned.

This invention relates to improvements in electrostatographic development devices and, particularly, to improvements wherein these devices are arranged to produce solid area coverage during the making of xerographic reproductions.

In conventional xerography, solid electrostatic images cannnot be reproduced on a xerographic selenium plate. When the surface charge is uniform in density over a large area, electric fields are present in the air space only near the edges of the charge pattern where the variation in surface charge density or potential differential exists. In the central portions, the eld is contained entirely within the dielectric or the photoconductive layer and, therefore, the central portion of such a charge pattern would exhibit no attraction for oppositely charged developer particles. Under these conditions, electric elds appear in the air space near the image surface where there are discontinuities or variations in surface charge density.

Therefore, the principal object of theinvention is to produce solid electrostatic image patterns in accordance with solid document areas to be reproduced. A further object of the invention is to maintain consistency in solid image quality during xerographic processing. Another object of the invention is to produce solid electrostatic images which can be transferred to any paper, Mylar, aluminum, brass, copper or4 zinc'plates or offset masters.

These and other objects of the invention are attained by means of the application of a fine mesh insulating material, such as silk, nylon, Daeron or the like to a xerographic plate as the same is being charged prior to the exposure step. The insulating material is removed prior to exposure thereby resulting in the presence of many areas of charge upon the plate. During the development step negatively charged toner particles are attracted to the microislands, so to speak, that are within the image areas thereby resulting in high quality solid area image reproduction.

A preferred form of the invention is shown in the accompanying drawings in which:

FIG. 1 is a schematic sectional view of a typical xerographic reproduction machine embodying the principles of the invention;

FIG. 2 is a fragmentary schematic view of the side of the xerographic drum used in FIG. 1 in conjunction with other elements of the machine; and,

FIGS. 3-5 are schematic illustrations of stages of image development according to the present invention.

For a general understanding of a typical xerographic processing system in which the invention may be incorporated, reference is made to FIG. 7 in which various components of a typical system are schematically illustrated. As in all xerographic systems, a light image of an orginal to be reproduced is projected onto the sensitized surface of a xerographic plate to form an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged developing material to form a xerographic powder image corresponding to the latent image on the plate surface. The powder image is then electrostatically transferred to a support surface such as a sheet of paper or the like to which it may be fused by a fusing device whereby the powder image is caused permanently to adhere to the support surface.

For purposes of the present disclosure, the xerographic reproduction machine includes an exposure station at which a light or radiation pattern of a document to be reproduced is projected by a lens 10 onto an electrostatographic surface, such as a xerographic drum 12.

The xerographic drum 12 is detachably secured to a shaft SH-l mounted in suitable bearings in the frame of the machine and is driven in a counterclockwise direction by a motor at a constant rate that is proportional to the scan rate for the document being reproduced whereby the peripheral rate of the drum surface is identical to the rate of movement of the projected light image of the document. The drum surface comprises a layer of photoconductive material on a conductive backing that is sensitized prior to exposure by means of a corona generating device 14.

The exposure of the drum to the document light image discharges the photoconductive layer in the areas struck by light, whereby there remains on the drum an electrostatic latent image in configuration corresponding to the light image projected from the document. As the drum surface continues its movement, the electrostatic latent image passes through a developing station A in which there is positioned a developer apparatus including a casing or housing 16 having a lower or sump portion for accumulating developing material. A bucket-type conveyor having a suitable driving means may be used to carry the developing material to the upper part of the developer housing where it is cascaded over a hopper chute onto the xerographic drum.

As the developing material is cascaded over the xerographic drum, toner particles are pulled away from the carrier component of the developing material and deposited on the drum to form powder images while the partially denuded carrier particles pass off the drum into the developer housing sump. As toner powder images are formed, additional toner particles must be supplied to the developing material in proportion to the amount of toner deposited on the drum.

Positioned next and adjacent to the developing station is the image transfer station B which includes a pair of rollers 18 for holding a support material in the form of paper web P against the surface of the drum to receive the developed xerographic powder image therefrom. The web P is moved in synchronism with the rotation of the drum by means of a take-up roll 20 which drives the support material P from a supply roll 22. A suitable drive mechanism (now shown) is connected to the drum 12 for imparting rotation thereto at a continuous speed. This drive mechanism may be connected to the take-up roll 20 for imparting rotation thereto thereby producing movement of the web material P in the same peripheral direction and at the same speed as the peripheral surface of the drum. In order to insure identical movement of the two co-acting surfaces, a Suitable programming device may be utilized to effect continuous synchronous movement of these surfaces.

The transfer of the xerographic powder image from the drum surface to the transfer material is effected by means of a corona transfer device 23 that is located at or immediately after the point of contact between the transfer material and the rotating drum. The corona transfer device 23 is substantially similar to the corona discharge device 14 in that it includes an array of one or more corona discharge electrodes that are ener-gized from a suitable high potential source and extend transversely across the drum surface and are substantially enclosed within a shielding member.

In operation the electrostatic field created by the corona discharge device is effective to attract the toner particles comprising the xerographic powder image from the drum surface and cause them to adhere electrostatically to the surface of the transfer material.

Immediately subsequent to the image transfer station, the transfer material is carried to a fixing device in the form of a fuser assembly 25 whereby the developed and transferred xerographic powder image on the sheet material P is permanently fixed thereto. After fusing, the finished copy is preferably discharged from the apparatus at a suitable point for collection externally of the apparatus.

The next and final station in the device is a drum cleaning station C having positioned therein a corona precleaning device similar to the corona charging device 14 but of opposite polarity to impose an electrostatic charge on the drum and residual powder adherent thereto to aid in effecting removal of the powder and a drum cleaning device in the form of rotary brushes 26 adapted to remove any powder remaining on the xerographic drum.

In general the electrostatic charging of the Xerographic drum in preparation for the exposure step and the electrostatic charging of the support surface to effect transfer are accomplished by means of corona generating devices whereby electrostatic charge on the order of from 700 to 800 volts is applied to the respective surface in each instance. Although any one of a number of types of corona generating devices may be used, a corona charging device of the type disclosed in Vyverberg Pat. No. 2,836,- 725 is used for both the corona charging device 14 and the corona transfer device 23, each of which is secured to suitable frame elements of the apparatus and connected to a suitable electrical circuit.

As shown in FIG. 1, the charging corotron 14 is surrounded by a nonconductive silk screen 30 or other ne insulating mesh material, such as nylon, Daeron or the like, which is arranged in the form of an endless web being supported on and driven by three rollers 31, 32 and 33. The axis of the rollers 31, 32 and 33 are arranged parallel to each other and to the shaft SH-1 for the drum 12. Each of the rollers 32 and 33 is supplied at their respective ends with O-rings (see FIG. 2) which are engageable with the periphery of the drum 12. The O-rings 34 are spaced so as to be positioned out of contact with the photoconductive material on the drum and are adapted to drive the rollers 32 and 33 during rotation of the drum for imparting movement of the silk material 30 at the same speed as the peripheral surface for the drum. As shown in FIG. l, the rollers 32 and 33 maintain a portion of the silk screen in intimate contact with the photoconductive surface on the drum.

The charging corotron 14, which for illustrative purposes is supplied with positive voltage, is mounted centrally above the portion of the intimate Contact zone between the silk screen and the photoconductor surface and is positioned between the rollers 32 and 33 and spaced the normal distance from the drum surface as is customary in xerographic machines. During charging of the xerographic drum with a portion of the silk screen upon the selenium surface of the duim, a positive charge is placed upon the individual fabric strands of the silk screen as well as upon the selenium surface in those portions thereof between the fabric strands. In FIG. 3 the fabric strands are indicated by the number 36 and the positive symbols are utilized to indicate positive charge placed upon the photoconductive surface 37 of the Xerographic drum and fabric strands. As the silk material is rolled away from the photoconductive surface by the roller 33, the silk screen material is removed from the drum surface taking with it the positive charges on the fabric strands 37. This results in the formation on the drum of a screen pattern of noncharged lines corresponding to the screen pattern of the material 30 and, also in a multiplicity of positively charged microislands 38 spread upon the entire surface of the photoconductor 37. Each of the islands 38 retains its positive charge thereby providing a potential differential along the perimeters thereof which, for those that remain after a light image charge pattern has lbeen produced during the exposure step, attracts negatively charged toner particles. After exposure, nonimage areas are discharged leaving solid areas comprising these tiny islands 38 surrounded by an electric field along the perimeter of each island. The image patterns or areas are then developed by the development mechanism 16 and the developed image is transferred to the paper web material P, as in the conventional process of xerography.

The size of the islands 38 will be determined by the mesh of the silk screen 30 and, preferably, the mesh should be relatively high in order that many small islands may be formed throughout the charging step of the xerographic drum. The light image areas resulting from exposure of a document by the lens system 10 will then comprise many of the charged microislands and, for relatively large solid areas of the image pattern, the microislands will break up the solid areas adapting them to solid area development during the development process of the image patterns. Suitable means not shown may be utilized to electrically ground out the positive charge which is placed upon the silk screen material 30 as the same is rolled away from the selenium drum. As in conventional xerography, the electrostatic image that is transferred to the web material P may also be transferred to Mylar, aluminum, brass, copper, zinc plated or offset masters.

While the invention as been described with reference to the arrangement disclosed herein, it is not confined to the details set forth, and their application is intended to cover such modifications or changes as may come within the purpose of the improvements for the scope of the following claims.

What is claimed is: 1. The method of producing and developing an electrostatic latent image on an electrostatic printing plate by exposure to image light rays of a document to be copied comprising in sequence:

placing a mesh element having a multiplicity of portions transparent to light rays against the electro-1 static plate prior to exposure to the image light rays and charging of the plate, said element being of a material capable of holding an electrostatic charge,

uniformly charging the plate with a voltage of one polarity through said element for producing a charge thereon and upon the portions of the plate which said transparent portions are placed,

removing the element thereby removing the charge thereon and effecting upon the plate a non-charged pattern and a multiplicity of charged areas, exposing the plate with image light rays of the document being copied for producing an electrostatic latent image in accordance with the document, and applying developer material, having a charge thereon of a polarity opposite that of said one polarity, to the electrostatic latent image.

2. The method of producing and developing an electrostatic latent image on an electrostatic printing plate by exposure to image light rays of a document to be copied comprising 1n sequence:

placing a silk screen material against the electrostatic plate prior to exposure to the image light rays and charging of the plate, said element being of a material capable of holding an electrostatic charge,

uniformly rcharging the plate with a voltage of one Ipolarity through said screen material for producing a charge thereon and upon the portions of the plate which said material is placed,

removing the material thereby removing the charge thereon and effecting upon the plate a non-charged pattern and a multiplicity of charged areas,

exposing the plate with image light rays of the document being copied `for producing an electrostatic latent image in accordance with the document,

and applying developer material, having a charge thereon of a polarity opposite that of said one polarity, to the electrostatic latent image.

3. The method of claim 2 wherein said plate is continuously moved and said placing of the screen material is performed at a Speed in synchronism with the movement of said plate.

4. The method of claim 2 wherein the screen is removed in synchronism with the placing of the screen material upon said plate.

5. The method of claim 2 wherein the screen material is arranged as an endless web around a plurality of rollers and positioned so as to place the material in contact with the plate.

References Cited UNITED STATES PATENTS 2,965,868 12/1960 Eichler. 3,152,528 10/1964 Pendry. 3,196,012 7/ 1965 Clark. 3,248,216 4/1966 Weigl. 3,281,857 10/1966` Kaiser. 3,339,469 9/1967 McFarlane. 3,449,568 6/1969 Vock Z50-49.5

GEORGE F. LESMES, Primary Examiner J. C. COOPER III, Assistant Examiner U.S. Cl. X.R. 

